Pretreatment of metal surfaces

ABSTRACT

Sheet metal whose surfaces have been freshly prepared chemically as by pickling and/or nickel coating, is covered with a coating of a water-soluble polyphosphate which may include an emulsifier and/or a water-soluble organic polymer, preferably film-forming. The coating protects the fresh surface during storage and/or shaping and the metal may thereafter be enamelled either with or without prior removal of the polyphosphate coating.

O United States Patent 11 1 1111 3,723,162

Leontaritis et al. 1 51 Mar. 27, 1973 54 PRETREATMENT 0F METAL 2,856,322 10/1958 Parson et a]. ..l48/6.15 R SURFACES 2,337,856 12 1943 Rice et al ..l48/6.15 R 3,050,406 8 1962 K 1 148 6.15 R X [75] Inventors Lamb Lwnmms, 2 327 885 811943 ..11 7 71 M x N'lmlaus lyeverkusen? Hans 3,468,724 9/1969 Reinhold ..l48/6.2X Hoffman", Lelchlmgen 3,222,291 12/1965 Heitetal ..252 3s9 y 2,742,369 4/1956 Hatch ..252/389 [73] Assignee: Farbenfabriken Bayer Aktien- 215151529 7/ Ryzmar ct "352/387 gesellscha, Leverkusen' Germany 3,151,087 9/1964 Ryzmar et al. ..252/387 [22] Filed: 1970 Primary ExaminerRalph S. Kendall 21 APPL 93,104 Att0rneyBurgess, Dinklage & Sprung [30] Foreign Application Priority Data [57] ABSTRACT Sheet metal whose surfaces have been freshly Dec. 30, 1969 Germany ..P I9 65 586.8 p p chemically as y p g and/or nickel coat [52] S Cl 7/71 M 117/127 117/49 ing, is covered with a coating of a water-soluble 72/46 25/1966 204/38 polyphosphate which may include an emulsifier and/or 1 17 32 C a water-soluble organic polymer, preferably film-form- 51 1m. (:1. ..c23r 11/00 s- The coating Protects the fresh Surface during 58 Field of Search ..ll7/6, 127, 71 M; l48/6.l5; storage and/9r p g and the metal y thereafter 252/387, 389 be enamelled either with or without prior removal of the polyphosphate coating. [56] References Cited v UNITED STATES PATENTS 9 Claims, No Drawings 3,278,328 10/1966 Okrent ..l17/47 R PRETREATMENT OF METAL SURFACES The invention relates to protecting freshly prepared metal surfaces.

The direct white enamelling of steel sheet has in recent years become a reliable production process. A consistent quality of the enamelled products combined with a reduction in the enamelling costs have been achieved by rationalizing the production program and introducing continuous, programmed production methods. Further simplification in the operational program in the enamelling plant is provided by the fact that the strip used for producing the blanks is already pretreated in the steel works to prepare it for the single layer enamelling, this pretreatment consisting e.g. of a suitable pickling process followed by nickel coating. The bonding surface thereby obtained which serves as a foundation for the single layer white enamelling must, of course, then be protected before the forming operation so that the layer applied to it will not be damaged. This protection can be achieved by the conventional process of pretreating the strips with an electrolytically applied layer of zinc (Belgian Patent specification No. 557,963 and French Patent specification No. 1,187,958).

The disadvantage of such a protective layer of metal lies in the complicated after-treatment which makes it necessary to set up installations consisting of several stages in the enamelling plant (degreasing, rinsing and removal of the protective layer by pickling with acid, e.g. hydrochloric acid, rinsing and neutralizing).

For the same reason, it is more rational if metal materials which are formed by drawing, pressing, pickling or other non-machining processes and are then provided with a firmly adhering metal or non-metallic layer are degreased and pickled immediately after they have been produced and are then provided with a covering which protects their surfaces but which can easily be removed. Metal coatings which are deposited by electrocoating or by chemical processes or stoving lacquers, dipping varnishes and other synthetic resin coatings also require a very clean foundation to ensure permanent protection of the surfaces. The problem of providing the metal worker with a starting material which is practically ready for use for the subsequent coating operation is, therefore, not confined to the enamelling industry.

The advantages are numerous. Not only is there a saving in material due to the elimination of the additional step of rust removal or the removal of other layers of corrosion but the pickling and the working up of the waste liquors which is associated with this can be carried out more economically in large plants.

It is accordingly an object of the invention to provide a pretreated coated metal surface which can directly be shaped and then provide with an adherent protective layer such as enamel, with or without removal of the coating.

This and other objects and advantages are realized in accordance with the present invention wherein the pretreated fresh surface of a metal article is coated with a film of a water-soluble polyphosphate.

The process according to the invention is used advantageously in enamelling work and is particularly efficient for single layer enamelling which requires specially pretreated metal surfaces.

The steel sheets which are to be shaped are advantageously degreased immediately after they have been produced, pickled and then covered with a layer of nickel. The surface treatment is carried out by conventional processes which may be either purely chemical, fully electrolytic or a combination of the two. Thus, for example, an electrolytically degreased, electrolytically pickled and electrolytically nickel plated metal sheet may first be cleaned by a coarse alkaline degreasing operation.

The sequence of pretreatment stages would in this case be as follows:

Coarse alkaline degreasing;

electrolytic precision degreasing;

rinsing;

electrolytic pickling;

rinsing;

electrolytic nickel plating;

rinsing; and

drying.

The advantage of the electrolytic pretreatment compared with the purely chemical method lies in a shortening of the total time of stay by about 65 percent. The water-soluble polyphosphate is thereafter applied, e.g. by the immersion or spray process. The entire treatment, including the production of the sheet metal, can be carried out completely continuously. The degreasing, pickling and nickel plating are advantageously carried out electrolytically. The sheet metal provided with the covering layer can then be shaped by rolling, pressing or drawing or by other non-machining methods. After the shaping operation, the covering may be stripped off or washed off with water before the enamelling process without the surfaces which have been prepared for enamelling being in any way impaired.

It has surprisingly been found that the covering of a polyphosphate used according to the invention acts as a lubricant in the shaping operation, especially if an organic auxiliary agent is added, and no impairment in the bonding of the enamel or in the enamel surface can be ascertained even if the covering is incompletely removed or not removed at all.

This gives rise to the interesting possibility of operating the presses without oil, so that after-treatment can be avoided. In practice, such a method of procedure will not always be possible since contamination with substances which interfere with the enamelling process can hardly be avoided. Nevertheless, it is very advantageous that a satisfactory enamelling can be ensured even if the protective layer has not been completely removed.

The process according to the invention is not confined to enamelling processes but can also be applied to similar technical fields in which shaped metal articles are subsequently provided with a protective layer; in all these cases, however, if the protective layer is to be firmly bonded, a carefully cleaned and in some cases specially pretreated surface is necessary. As already mentioned, the process affords advantages also for the production, for example, of metal articles which, after they have been shaped, are treated with an electrocoating or chemical process to metallize them or cover them with a synthetic resin coating.

Polyphosphates which are suitable for the process of the invention are salts of polyphosphoric acids having the general formula: HO[HOPO H in which n is an integer of at least 10, and preferably more than 100. The hydrogen is replaced by monovalent cations such as Na, K, Li", NHf, or substituted ammonium ions; both polyphosphates which have only one type of ion and polyphosphates with different ions may be used. In the case of high molecular weight polyphosphoric acids, mixed salts such as Na/K salts are preferably used for the sake of solubility in a polar, preferably aqueous, medium.

The polyphosphate solutions used for producing the protective layers according to the invention are prepared by conventional methods. The starting materials used are generally polyphosphates such as Kurolls salts, Madrells salts or Graham's salts, and, if they are insufficiently soluble in water, some of the cations are replaced by other types of cations. For example, K-Kurollate (K P which is insoluble in water may be suspended in water and then brought into solution by the addition of from 0.5 to 6 parts of a sodium phosphate such as sodium polyphosphate [NaPO or the phosphates Na P O Na P O (NaPOQ (sodium trimetaphosphate), (NaPO (sodium hexametaphosphate) and Na PO The potassium polyphosphate may also be dispersed in water and some of the potassium ions may be replaced by sodium ions by means of an ion exchanger which contains sodium ions so that a soluble Na/K polyphosphate is obtained.

Numerous other salts of monovalent cations may, of course, also be reacted with polyphosphates of the composition [K P0 or [NaPO;,],,, and the polymer molecules may then still be insoluble even after some of the cations have been replaced by other cations; this insolubility is in most cases due to the presence of foreign ions in the supernatent solution. Gradual solution is then achieved by decanting and adding salt-free water. The solutions which contain little or no foreign salt and which generally have elevated viscosities may be used for the process of the invention as may also polyphosphate solutions which contain foreign salts.

It has surprisingly been found that the properties of the protective layers of polyphosphates can be influenced by the addition of organic substances of high or low molecular weight. The addition of water-soluble polymers of medium to low molecular weights may have a plasticizing effect which makes the coatings pliable, elastic and smooth and have a lubricating effect in the shaping operation carried out on sheet metal. By adding a low molecular weight substance, for example, the rate at which the protective coating dissolves when it is being removed prior to the enamelling can be increased.

The following are mentioned as examples of substances which have an important effect in influencing the physical properties of the protective layers, such as their strength, elasticity and lubricity: polyethylene oxide and polyglycol ethers, polypropylene glycol, polyvinyl alcohol, polyvinyl ether, polyvinyl pyrrolidone, polyacrylamides, polyacrylates, maleic acid anhydride copolymers modified e.g. with styrene and isobutylene, and water-soluble polyesters, as well as mixtures of such water-soluble polymers and copolymers. The effect on the films varies according to the molecular weight of the organic polymers added, the films becoming softer and more ductile when polymers of low molecular weight are added, whereas the addition of high molecular weight polymers influences the properties of the protective layers in the direction of the film properties of the organic polymers.

Substances which although influencing the physical properties of the protective layers mainly exert their influence on the redissolution of the protective layers are mainly those which have a surface active effect, such as non-ionic, anionic or cationic emulsifiers. The following are mentioned as examples from this group of substances: polyglycol ethers, reaction products of phenols or alcohols with ethylene oxide, fatty acid salts, sorbitan esters, sulfonates such as alkylbenzene sulfonates and sulfuric acid semiesters of long chain alcohols. The rate of solution is, of course, also increased by the addition of oligomers or polymers of low molecular weight. The organic substances may, in principle, be added in any proportion, based on the polyphosphate. When adding organic polymers or plasticizers, the desired effects are generally achieved with quantities of from about 1 to percent, whereas the emulsifiers are sometimes effective in very small quantities, i.e. in concentrations of from about 0.01 to 10%, based on the polyphosphate. One may, of course, add both organic polymers and emulsifier-type compounds. The polyphosphates and the organic additives may be applied separately, in which case mixing may occur due to diffusion in the applied layers of solutions. Preferably, however, the solutions of the polyphosphates to be applied and the organic additives are first mixed. When using mixtures of polyphosphates and organic substances, small quantities of age resistors or stabilizers for the organic substances may be added.

The coatings with the substances used according to the invention may be applied by conventional methods, e.g. by spraying, immersion and brush application. To apply very thin films, e.g. just a few molecules thick which corresponds to a surface coating of from 0.2 to 2 g of polyphosphate per m, it is advantageous to spray an aqueous or organic solution on to the surface and evaporate the solvent.

The process according to the invention is illustrated in more detail in the following examples.

EXAMPLE 1 A cold rolled steel sheet having the following approximate analytical composition:

C 0.004 MN=0.320 S=0.035 P=0.040

Si=0.020 Cu=0.027

was degreased with alkali, rinsed with warm and then cold water, pickled for 6 minutes in 9% H at 75 C, rinsed, nickel coated in a 1.2% NiSO -7 H 0 solution at 70 C and pH 3 for 6 minutes, rinsed with water and finally neutralized. The pretreated sheet was then coated with a 0.35 percent aqueous K-, Napolyphosphate solution by spraying. The coated metal sheet after drying, was then shaped, using an oil-free tool, and finally enamelled by direct enamelling with a B-Ti white enamel of the following composition:

38 SiO 15 alkali metal oxide 2 fluorine l9 TiO 0.5% MgO 3 P 0 without previous removal of the polyphosphate layer.

After the usual stoving at 820 C, the enamelling showed very good bonding of the enamel and a very good enamel surface.

The polyphosphate solution was obtained by reacting (KPO with NaCl in the molar ration of 1:3 by adding NaCl in a proportion of 1:3 to a suspension of (KPO in water, separating the swelled precipitate after stirring for one hour, and dissolving the precipitate in pure water.

The molecular weight of the polyphosphate is expressed in terms of its viscosity lrmr/c where c concentration in g/lOO ml of solvent.

lnnr/c 30 Determination in water 25 C and pH 7,

concentration 0.05 percent in the presence of 0.1 percent NaCl at 1- 0.98 dyn/cm The polyphosphate contains 7.5 percent of potassium and 13.1 percent of sodium.

EXAMPLE 2 A steel sheet having the analytical composition indicated in Example 1 was subjected to a preliminary alkaline degreasing operation, then thoroughly degreased electrolytically, rinsed, pickled electrolytically in a solution of 150 g of Na,SO -l0 11 0/1 and 5 ml of concentrated H 80 per liter at a current density of lOA/dm for 1 minute, rinsed and finally nickel coated in a solution of 80 g of NiSO -7 l'l O/l, 10 g of Nl-LCl/l, g of MgSO -7 H O/l and 5 g of H BO /l at a current density of0.5 A/dm for 1 minute.

After rinsing, the sheet was coated with a 0.35 percent aqueous K-, Na-polyphosphate solution by immersion, dried and then shaped in a hydraulic press as conventionally used in enamelling works. After the shaping operation, the protective layer was removed by spraying with water and the article was dried and enamelled by direct white enamelling with a B-Ti white enamel of the composition described in Example 1. Both adherence and enamel surface were excellent. The same polyphosphate solution as in Example 1 was used.

EXAMPLE 3 A steel sheet having the analytical composition indicated in Example 1 was pretreated as in Example 1. The pretreated sheet was brush coated with a 2 percent aqueous K-, Na-polyphosphate solution which contained 6 percent of K, Na-hexametaphosphate as foreign salt. After the layer which had been applied was dried, the sheet was subjected to a shaping operation as conventionally carried out in enamelling works. The polyphosphate layer was then removed by spraying with water. Following this, the shaped sheet was enamelled by direct white enamelling with a B-Ti white enamel of the composition given in Example 1. Both adherence and enamel surface were excellent. The polyphosphate solution was obtained by stirring [K- P0 with (NaPO l :3) in water. Viscosity lnnr/c 9.1

EXAMPLE 4 A steel sheet with the analytical composition indicated in Example 1 was pretreated as in Example 1. The pretreated sheet was spray coated with a 2 percent aqueous solution of K-, Na-polyphosphate solution which contained 5 percent of polyethylene glycol (molecular weight 1000) as plasticizer. After drying of the applied layer, the sheet was subjected to a powerful shaping operation in the enamelling works. The polymer coating was then removed by brushing under running water. The sheet was then enamelled by direct white enamelling with a B-Ti white enamel of the composition described in Example 1. Both adherence and enamel surface were excellent. The same K-, Napolyphosphate solution as in Example 3 was used.

EXAMPLE 5 A steel sheet with the analytical composition indicated in Example 1 was pretreated as in Example 2. The pretreated sheet was spray coated with a 2 percent aqueous 14-, Na-polyphosphate solution which contained 6 percent of K-Na-hexametaphosphate as foreign salt and 8 percent of polyvinyl ethyl ether (molecular weight 1,500) as plasticizer. After drying of the applied layer, the sheet was subjected to a powerful shaping operation in the enamelling plant. The polyphosphate layer was then removed by spraying with water. The subsequent direct white enamelling with a B-Ti white enamel of the composition described in Example 1 yielded excellent results as regards adherence and enamel surface. The polyphosphate solution was the same as that used in Example 3.

EXAMPLE 6 A steel sheet with the analytical composition indicated in Example 1 was pretreated as in Example 1. The pretreated sheet was coated with a 2 percent aqueous K-, Na-polyphosphate solution containing 0.8 percent of oleic acid oxethane sulfonate (Na-salt of the sulfuric acid monoester with olcyl alcohol), using two rubber rollers. After drying of the applied layer, the sheet was pressed in the enamelling plant to form a cover plate of an electric stove available on the'market. The polymer coating was then removed by spraying with water. It was found that this coating could be washed off exceptionally quickly.

Direct white enamelling using a B-Ti white enamel of the above composition was then carried out. Both the adherence and the enamel surface were excellent.

EXAMPLE 7 Three thin metal sheets A, B and C having the analytical composition indicated in Example 1 were pretreated as described'in Example 2. Sheet A was shaped into the cover plate of an electric stove in the enamelling plant without a protective layer of polymer. The shaped metal sheet A was enamelled by direct white enamelling with a B-Ti white enamel of the composition described in Example 1. Both the adherence and the enamel surface were very poor.

Sheet B was shaped exactly like sheet A but subsequently sprayed with water.

Direct white enamelling with the B-Ti white enamel of the above composition yielded poor results as regards the adherence and the enamel surface.

Sheet C was spray coated with a 2 percent K-, Napolyphosphate solution (see Example 3) containing l.5 percent of nonylphenol polyglycol ether before it was shaped. After drying of the polymer layer, sheet C was shaped in the same way as sheets A and B. The polyphosphate layer was then removed by spraying with water. The shaped metal sheet C was then direct white enamelled with the B-Ti enamel of the composition described in Example 1. Both the adherence and the enamel surface were excellent, in contrast to sheets A and B.

EXAMPLE 8 A steel sheet having the analytical composition indicated in Example 1 was pretreated as in Example 1. The pretreated sheet was spray coated with a 2 percent aqueous K-, Na-polyphosphate solution which contained percent of polyethylene gylcol (molecular weight 1,000) as plasticizer. The metal sheet was subjected to powerful shaping in the enamelling plant without previous drying of the applied layer. The polymer coating was then removed by brushing under running water. Direct white enamelling with a B-Ti white enamel of the composition described in Example 1 was then carried out. Both the adherence and the enamel surface were excellent. The same K-, Napolyphosphate solution as in Example 3 was used.

It will be appreciated that the instant specification and examples are set forth by way of illustration and not limitation, and that various modifications and changes may be made without departing from the spirit and scope of the present invention.

What is claimed is:

l. A process for the temporary protection of a metal surface which has been freshly prepared which comprises covering said surface with a coating of at least about 0.2 grams per square meter of a water-soluble mixed sodium and potassium salt of a polyphosphoric acid of the formula HO(HOPO ),,H in which n is at least about 10, whereby said coating may be removed by water to expose the fresh metal surface.

2. A process according to claim 1, wherein n is at least about 100.

3. A process according to claim 1, wherein the polyphosphate coating includes an organic water-soluble polymer, which polymer serves to plasticize the poly-phosphate.

4. A process according to claim 3, wherein said organic water-soluble polymer is itself film-forming.

5. A process according to claim 1, wherein the polyphosphate film includes an emulsifier.

6. A process according to claim 1, wherein n is at least about 100, and wherein said polyphosphate coating includes an emulsifier and a film-forming organic water-soluble polymer.

7. In the production of a shaped metal structure having an adherent protective layer, comprising the steps of chemically or electrolytically pretreating a metal sheet to provide a fresh surface, shaping said sheet into a structure and providing an adherent, protective layer on said shaped structure, the improvement which comprises covering said fresh surface with a coating of at least about 0.2 grams per square meter of water-soluble mixed sodium and potassium salt of a polyphosphoric acid of the formula HO(H OPO ),,I-l in which n is at least about 10, before shaping w hereby said fresh surface is unimpaired during shaping and is directly ready for receipt of the adherent protective layer.

8. A metal structure having a freshly prepared chemically or electrolytically treated surface and a protective coating of at least about 0.2 grams per square meter of a water-soluble mixed sodium and potassium salt of a polyphosphoric acid of the formula HO(HOPO in which n is at least about 10.

9. A steel sheet according to claim 8, wherein said pretreatment is a nickel coating, and said protective coating is a mixed sodium-potassium salt of a polyphosphoric acid of the formula HO(HOPO ),,H wherein n is at least about 100, said coating including an emulsifier and a film-forming organic water-soluble polymer. 

2. A process according to claim 1, wherein n is at least about
 100. 3. A process according to claim 1, wherein the polyphosphate coating includes an organic water-soluble polymer, which polymer serves to plasticize the poly-phosphate.
 4. A process according to claim 3, wherein said organic water-soluble polymer is itself film-forming.
 5. A process according to claim 1, wherein the polyphosphate film includes an emulsifier.
 6. A process according to claim 1, wherein n is at least about 100, and wherein said polyphosphate coating includes an emulsifier aNd a film-forming organic water-soluble polymer.
 7. In the production of a shaped metal structure having an adherent protective layer, comprising the steps of chemically or electrolytically pretreating a metal sheet to provide a fresh surface, shaping said sheet into a structure and providing an adherent, protective layer on said shaped structure, the improvement which comprises covering said fresh surface with a coating of at least about 0.2 grams per square meter of water-soluble mixed sodium and potassium salt of a polyphosphoric acid of the formula HO(HOPO2)nH in which n is at least about 10, before shaping whereby said fresh surface is unimpaired during shaping and is directly ready for receipt of the adherent protective layer.
 8. A metal structure having a freshly prepared chemically or electrolytically treated surface and a protective coating of at least about 0.2 grams per square meter of a water-soluble mixed sodium and potassium salt of a polyphosphoric acid of the formula HO(HOPO2)nH in which n is at least about
 10. 9. A steel sheet according to claim 8, wherein said pretreatment is a nickel coating, and said protective coating is a mixed sodium-potassium salt of a polyphosphoric acid of the formula HO(HOPO2)nH wherein n is at least about 100, said coating including an emulsifier and a film-forming organic water-soluble polymer. 